1. Field of the Invention
The present invention relates to a solid-state imaging device, a manufacturing method thereof, and a camera. In particular, the present invention relates to a solid-state imaging device in which pixels having photodiodes are arranged on a light sensing surface in the form of a matrix, a manufacturing method thereof, and a camera provided with the corresponding solid-state imaging device.
Also, the present invention relates to a solid-state imaging device and an electronic device. In particular, the present invention relates to a solid-state imaging device, in which a photoelectric conversion unit and a pixel transistor that outputs a signal charge generated by the photoelectric conversion unit as an electric signal are installed in a pixel area and an electronic device.
2. Description of the Related Art
Typically, a charge accumulation capacity of a photodiode greatly depends upon the capacity of a PN junction formed in the vicinity of a substrate surface. However, if pixel refinement is performed, the surface area of the photodiode becomes smaller to cause the area of the PN junction to become also smaller, and thus the charge accumulation capacity is reduced.
When a large quantity of light is incident to the photodiode, electrons photoelectrically converted in the photodiode easily overflow from the photodiode, and an image turns white.
Accordingly, if the pixel finement is performed, the dynamic range of the imaging device becomes small.
Accordingly, in order to enlarge the charge accumulation capacity, it is necessary to increase the capacity of the PN junction of the photodiode. For this, it is important to increase the junction capacity by steepening a gradient of an effective impurity concentration in the PN junction.
In order to obtain a steep PN junction, a shallow and dense ion implantation is performed, and then heat treatment should be considered to prevent thermal diffusion.
However, if the heat treatment is insufficient, implantation defects caused by the ion implantation are not removed by sufficient heat treatment, but remain in the vicinity of the PN junction.
Also, even in an etching process such as reactive ion etching or the like in a gate etching and side wall etch back, defects and impurities are introduced in the vicinity of the PN junction.
However, since the heat treatment should be reduced for the above-described reasons, it may not be possible to perform the heat treatment in order to sufficiently remove of the defects.
Accordingly, in the case of attempting to increase the junction capacity through heightened steepness of the PN junction on the surface, the above-described defects remain in the vicinity of the PN junction, and transition occurs between trap assist bands. Due to this, even in the case where the transition occurs between bands dominated only by the original electric field intensity, a large amount of junction leak current is induced, which results in an undesirable increase of dark current.
As described above, improving the dynamic range by simply steepening the PN junction causes yield deterioration such as the increase of dark current or the like.
Japanese Unexamined Patent Application Publication No. 2005-167588 describes that the vicinity of an interface of SiO2/Si is pinned by burying P+ polysilicon in an element isolation area and applying a negative potential to the polysilicon. The pinning of the P+ polysilicon is limited to the inside of the element isolation area.
Japanese Unexamined Patent Application Publication No. 2001-189286 describes that a Si active layer is deposited on a glass substrate and a photo gate Al is arranged thereon. The photo gate is used to form a depletion layer in order to accumulate in the active layer one-side charge of a carrier that is generated by the photoelectric conversion in the active layer.
Japanese Unexamined Patent Application Publication No. 2003-338615 describes that a transparent electrode is arranged on the back surface, and a negative potential is applied thereto. The transparent electrode is for the pinning of the back surface.
Japanese Unexamined Patent Application Publication No. 2003-31785 describes a CMOS image sensor irradiated with light from the back surface.
Japanese Unexamined Patent Application Publication Nos. 2006-173351 and 2007-258684 and International Patent Publication No. 2008/139644 describe configurations in which an electrode is installed on the front surface of a light sensing surface.
An electronic device, such as a digital video camera, a digital still camera, or the like, includes a solid-state imaging device. For example, the electronic device includes a CMOS (Complementary Metal Oxide Semiconductor) type image sensor, a CCD (Charge Coupled Device) image sensor, or the like, as the solid-state imaging device.
According to the solid-state imaging device, a plurality of pixels is arranged on the front surface of a semiconductor substrate. In each pixel, a photoelectric conversion unit is installed. The photoelectric conversion unit, for example, is a photodiode, which receives light incident through an optical system attached to the outside through a light sensing surface, performs a photoelectric conversion of the received light, and generate a signal charge.
According to the CMOS type image sensor among the solid-state imaging devices, a pixel is configured so that the CMOS type image sensor includes a pixel transistor in addition to the photoelectric conversion unit. The pixel transistor includes a plurality of transistors to read the signal charge generated by the photoelectric conversion unit and to output the read signal charge on a signal line as an electric signal.
As the solid-state imaging device, a “front-illuminated”, in which the photoelectric conversion unit receives the light incident from the front-surface side on which a circuit element, an interconnection, or the like is installed in a semiconductor substrate, is known. In the case of the “front-illuminated”, since the incident light is shielded or reflected by the circuit element, interconnection, or the like, it may be difficult to improve the sensitivity. As a result, a “back-illuminated”, in which the photoelectric conversion unit receives the light incident from the back-surface that is opposite to the surface on which the circuit element, interconnection, or the like, in the semiconductor substrate is installed, has been proposed (for example, see Japanese Patent No. 3759435).
In the solid-state imaging device as described above, an effective pixel area and an optical black area are installed on the surface of the semiconductor substrate. In the effective pixel area, effective pixels, in which the photoelectric conversion unit receives the incident light, are arranged. The optical black area is installed in a portion surrounding the effective pixel area, and in the optical black area, optical black (OB) pixels, in which a light shielding layer for shielding the light incident to the photoelectric conversion unit is installed, are arranged. From the OB pixel, a reference signal of a black level is output. Also, in the solid-state imaging device, a process of correcting signals output from the effective pixels based on signals output from the OB pixels is performed to remove noise components such as a dark current and the like (for example, see Japanese Unexamined Patent Application Publication Nos. 2006-147816, 2005-101985, 2009-164247, and 2006-25147).
In addition, in order to prevent optical crosstalk in the solid-state imaging device, it has been proposed to install a dummy pixel area between the effective pixel area and the optical black area. In the dummy pixel area, dummy pixels that are not connected to a read column circuit are installed to absorb signal charges leaking from the effective pixel area. Also, it has been proposed to install a conductivity type well, as a dummy pixel area, that is different from the well of the effective pixel area and to compulsively discharge the excessive charges in a bulk (for example, see Japanese Unexamined Patent Application Publication No. 2000-196055).